Heparin-binding epidermal growth factor-like growth factor, or HB-EGF, is a growth factor belonging to the EGF ligand family. HB-EGF gene-null knockout mice exhibit very detrimental phenotypes, such as cardiac function failure accompanied by cardiohypertrophy, and quickly die after birth (Nonpatent Reference 1). This shows that HB-EGF makes a profound contribution to the formation of the heart during gestation. In the adult, on the other hand, its expression is distributed across a relatively broad range of tissues, e.g., the lung, heart, brain, and skeletal muscle (Nonpatent Reference 2), and HB-EGF has a very important role not just during gestation, but also in maintaining biological function in the adult (Nonpatent Reference 3).
HB-EGF occurs as two different structures in vivo: a membrane-bound HB-EGF that is expressed on the cell surface of HB-EGF-expressing cells (designated below as proHB-EGF) and a secreted-form HB-EGF that occurs free from the cell (designated below as sHB-EGF or active-form HB-EGF). The structures of proHB-EGF and sHB-EGF are shown schematically in FIG. 1. The proHB-EGF precursor protein is composed of 208 amino acids and is composed, considered from the N-terminal, of a signal peptide, propeptide, heparin-binding domain, EGF-like domain, juxtamembrane domain, transmembrane domain, and cytoplasmic domain. Cleavage of the signal peptide from the proHB-EGF precursor protein results in the expression of proHB-EGF as a type 1 transmembrane protein. Subsequently, proHB-EGF is subjected to protease digestion, known as ectodomain shedding, and sHB-EGF, composed of 73 to 87 amino acid residues, is released into the extracellular environment. This sHB-EGF is composed of just two domains, the heparin-binding domain and the EGF-like domain, and binds as an active ligand to the EGF receptor (Her1) and EGF receptor 4 (Her4). This results in the induction of proliferation, via the downstream ERK/MAPK signaling pathway, in a variety of cells, e.g., NIH3T3 cells, smooth muscle cells, epithelial cells, keratinocytes, renal tubule cells, and so forth (Nonpatent Reference 4). A substantial reduction in proliferation ability occurs with cells that express only proHB-EGF due to the introduction of mutation into the region that participates in ectodomain shedding. In addition, transgenic mice that express only proHB-EGF have the same phenotype as HB-EGF knockout mice. Based on these observations, the function of HB-EGF as a growth factor is thought to be borne mainly by the secreted form of HB-EGF (Nonpatent References 5 and 6).
proHB-EGF, on the other hand, is also known to have a unique function in vivo different from that of sHB-EGF. That is, proHB-EGF was initially known to function as a receptor for the diphtheria toxin (DT) (Nonpatent References 7 and 8). However, subsequent research demonstrated that proHB-EGF forms complexes at the cell surface with molecules such as DRAP27/CD9 and also integrin α3β1 and heparin sulfate and participates in cell adhesion and migration. Operating through the EGF receptor (designated hereafter as EGFR) via a juxtacrine mechanism, proHB-EGF has also been shown to inhibit the growth of neighboring cells and to induce neighboring cell death. Thus, with regard to HB-EGF in its role as a ligand for EGFR, the membrane-bound proHB-EGF and secreted-form sHB-EGF are known to transmit diametrically opposite signals (Nonpatent References 5 and 8).
HB-EGF has a strong promoting activity on cell proliferation, cell movement, and infiltration in a variety of cell lines, for example, cancer cells. In addition, an increase in HB-EGF expression over that in normal tissue has been reported for a broad range of cancer types (e.g., pancreatic cancer, liver cancer, esophageal cancer, melanoma, colorectal cancer, gastric cancer, ovarian cancer, bladder cancer, and brain tumors), suggesting that HB-EGF is strongly implicated in cancer proliferation or malignant transformation (Nonpatent References 4 and 10).
Based on these findings, the inhibition of cancer cell growth via an inhibition of HB-EGF activity has therefore been pursued. The following effects, inter alia, have been reported for efforts to inhibit the action of HB-EGF using anti-HB-EGF neutralizing antibodies: an inhibition of DNA synthesis in 3T3 cells (Nonpatent Reference 11), an inhibition of keratinocyte growth (Nonpatent Reference 12), an inhibition of glioma cell growth (Nonpatent Reference 13), and an inhibition of DNA synthesis in myeloma cells (Nonpatent Reference 14).
The use of an attenuated diphtheria toxin (CRM197) that specifically binds to HB-EGF as an HB-EGF inhibitor has also been pursued. In fact, in a test of the efficacy in a mouse xenograft model (transplantation of an ovarian cancer cell line), the group receiving CRM197 presented a superior tumor shrinkage effect (Nonpatent Reference 15). In addition, clinical testing with CRM197 has also been carried out in cancer patients (Nonpatent Reference 6).
Thus, HB-EGF is clearly useful as a target molecule for anti-cancer agents, and the efficacy of HB-EGF inhibitor molecules such as CRM197 have in fact also been tested to date. However, CRM197 is a toxin not naturally present in the human body, and it is therefore thought that the clinical utilization of CRM197 will be encumbered by very substantial problems arising not just from its toxicity, but also from its antigenicity.
In addition, although neutralizing antibodies that can inhibit the activity of HB-EGF have in fact existed for some time as noted above, all of these have been polyclonal antibodies purified from goat antisera and hence cannot be used clinically. Thus, there is demand in the medical community for HB-EGF-neutralizing monoclonal antibodies that can exhibit a high neutralizing activity and that can realize the humanization and high production levels required for clinical applications.
However, when contemplating the clinical application of anti-HB-EGF neutralizing antibodies, toxicity such as antibody-dependent cell-mediated cytotoxicity (abbreviated below as ADCC activity) mediated by the antibody and effector cells and complement-dependent cytotoxicity (abbreviated below as CDC) is a risk due to the fact that, as cited above, HB-EGF is also expressed in vivo in a broad range of normal tissues in the form of proHB-EGF, which is the HB-EGF protein on the cell surface of HB-EGF-expressing cells. Other problems that must be addressed are the reduction in the efficiency of antibody accumulation at the tumor tissue and the reduction in blood concentration brought about by uptake of the antibody in normal tissues.
The references cited in this specification is listed below. The contents of these documents are herein incorporated by reference in their entirety. None of these documents is admitted as prior art to the present invention:
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